Process for the preparation of a supported iridium catalyst

ABSTRACT

A process for the preparation of a supported iridium catalyst by the contacting of, for example, an alumina support with a liquid medium containing a compound of iridium while maintaining the iridium-containing liquid medium at a pH of from 3 to 6 with an ammonium hydroxide or lower alkyl amine base. The catalyst is particularly useful in a reforming process.

United States Patent Erickson Feb. 18, 1975 [5 PROCESS FOR THEPREPARATION OF A 3,554,902 1/1971 Buss 252/466 PT SUPPORTED [RIDIUMCATALYST 3,730,909 5/1973 Armstrong et al. 252/466 PT 3,761,428 9/1973Sugier et a]. 252/466 PT [75] Inventor: Henry Erickson, Park Forest,[11.

[73] Assignee: Atlantic Richfield Company, New Primary Examiner-ArthurP. Demers York, NY. Attorney, Agent, or Firm-Th0mas J. Clough [22]Filed: Apr. 19, 1973 [21] App]. No.: 352,809 ABSTRACT A process for thepreparation of a supported iridium [52] us. Cl. 252/466 PT, 208/138catalyst by the Contacting for example an alumina 51 Int. Cl B01j 11/08PP with a quid medium Containing a Compound [58] Field of Search 252/466PT of iridium While maintaining the iridium-Containing liquid mediumat apH of from 3 to 6 with an ammo- [56] References Cited nium hydroxide orlower alkyl amine base. The cata- UNITED STATES PATENTS lyst isparticularly useful in a reforming process.

2,635,080 4/1953 Appell 252/466 PT 10 Claims, N0 Drawings PROCESS FORTHE PREPARATION OF A SUPPORTED IRIDIUM CATALYST This invention relatesto a process for the preparation of a supported iridium catalyst and,more particularly, to a process which provides for improved distributionof iridium on an inorganic oxide support.

Catalysts comprising platinum, for example, platinum-on-alumina, arewell known and widely used for reforming of naphthas in order to producehigh octane gasolines. Iridium has been proposed for use in catalyticreforming as a promoter for platinum-containing catalysts. Thus, US.Pat. No. 2,848,377 discloses reforming with a catalyst consisting ofplatinum and iridium on an alumina support. ln US. Pat. No. 3,554,902, aparticular reforming process is also disclosed using a platinum andiridium supported catalyst similar to the catalyst of US. Pat. No.2,848,377.

A particular problem associated with preparing supported iridiumcatalyst is to obtain a substantial homogeneous dispersion or iridium onthe support carrier. Thus, in Netherlands patent application No.71/02303, a process for the preparation of a platinum-iridium supportedcatalyst is disclosed which improves upon the catalyst methods ofmanufacture disclosed in U.S. Pat. No. 2,848,337. As set forth in theNetherlands application, one of the problems associated with thepreparation of a platinum-iridium supported catalyst is the referred toselective absorption phenomena which occurs during the impregnation ofthe carrier by the solution containing the active catalyst elements, forexample, platinum and iridium, For example, the generally used ammoniumsalts such as the chlorophatinate or the other salts of ammonium or thechloroiridate, lead to the formation of agglomerates of the metals thereby producing catalysts having significant activity loss.

It is an object of this invention to provide an improved process for thepreparation of a supported iridium reforming catalyst. Other objectswill be apparent from the following description.

It has now been found that an improved supported iridium catalyst can beobtained by process which comprises impregnating an alumina-containingsupport with a liquid medium containing a decomposable solubilizediridium compound, maintaining the pH of the liquid medium duringimpregnation of the support in the range of from 3 to 6, preferably fromabout 3.5 to about 5, with a base selected from ammonium hydroxide, alower alkyl amine, and mixtures thereof to yield an iridium-aluminacomposite comprising from about 0.01 to about 1 weight of iridium andsubjecting the resulting iridium-alumina composite to drying,calcination and reduction.

In the process of this invention, the aluminacontaining support isimpregnated with the iridiumcontaining liquid medium preferablycontaining a platinum or palladium metal component, preferably platinum.The metals are present in the liquid medium, i.e., as in an aqueoussolution, at a sufficient concentration to provide the desired quantityof iridium and platinum metal in the finished catalyst. In general, thetemperature of impregnation is from about ambient to about 212F., morepreferably from about 140F. to about 200F. The temperature of the liquidmedium is maintained at a level sufficient to solubilize substantiallyall of the iridium compound, preferably all of the iridium compound. Theimpregnation of the aluminacontaining support can be at a pressure offrom about 0.01 to about atmospheres, more preferably from about 0.01 toabout O.l atmospheres. The use ofa pressure below atmospheric pressureis particularly advantageous in the process of this invention. It isessential that the liquid medium is adjusted to a pH of from about 3 t06 prior to contact with the aluminacontaining support. The basematerial, e.g., ammonium hydroxide and/or lower alkyl amine, is added tothe liquid medium at a concentration sufficient to provide the requisitepH for the liquid medium. As set forth above, it is preferred to useco-impregnation of the aluminacontaining support with the iridium andplatinum compounds. However, it is contemplated within the scope of thisinvention that the alumina-containing support can be a catalystcomprising platinum-on-alumina. In such cases, the platinum in generalexists in the reduced form. It is, therefore, preferred that theplatinum-on-alumina containing support be subjected to oxidizingconditions followed by the impregnation with the iridium compoundutilizing the process of this invention. Alternatively, sequentialimpregnation can be utilized to prepare the iridium composite witheither type metal compound being utilized as the first metal. If themetals are sequentially added to the alumina support as, for example, bysequential impregnation, the carrier containing the first metal compoundadded can be dried and calcined prior to addition of the second metalcompound. Thereafter, the carrier containing both metals can be driedand calcined. In any event, the iridium-alumina containing composite isprepared according to the process of this invention.

The final unreduced catalyst, prepared by a method set forth above, isgenerally dried at a temperature of from about 200F. to about 600F. fora period of from about 2 to 24 hours or more and finally calcined at atemperature of about 700F. to about l500F., preferably from about 850F.to about 1300F. for a period of from about 1 hour to about 20 hours andpreferably from about 1 hour to about 5 hours.

The resultant calcined catalyst is generally subjected to reductionprior to use in the conversion of hydrocarbons. This step as previouslynoted is designed to insure chemical reduction of at least a portion ofthe metallic components.

The reducing media in general contacts the calcined catalyst at atemperature of about 800F. to about l200F., at a pressure in the rangefrom about 0 psig. to about 500 psig. and for a period of time of about0.5 to 10 hours or more and in any event, for a time which is effectiveto chemically reduce at least a portion, preferably a major portion, ofeach of the metallic components, i.e., platinum or palladium metal andiridium component, of the catalyst. By chemical reduction is meant thelowering of oxidation states of the metallic components below theoxidation state of the metallic component in the unreduced catalyst. Forexample, the unreduced catalyst may contain iridium salts in which theiridium has an oxidation state which can be lowered or even reduced toelemental iridium by contacting the unreduced catalyst with hydrogen.This reduction treatment is preferably performed in situ (i.e., in thereaction zone in which it is to be used), as part of a start-upoperation using fresh unreduced catalyst.

The essential constituent of the catalyst is iridium. This component maybe present as an elemental metal, as a chemical compound, such as theoxide, sulfide or halide, or in a physical or chemical association withthe inorganic oxide support and/or the other components of the catalyst.Generally, the iridium component is utilized in an amount whichresultsin a catalyst composite containing from about 0.01% to about 1%,preferably from about 0.025% to about 0.50%, by weight of iridiumcalculated as the elemental metal. The iridium component may beincorporated in the catalyst at any stage in the preparation of thecatalyst. As stated above, the procedure for incorporating the iridiumcomponent involves the impregnation of the support or its precursor,either in the form of a sol or gel, before, during or after the timeplatinum or palladium or mixtures thereof are added. The impregnationsolution is, in general, an aqueous solution suitable solubilizediridium salt such as chloroiridic acid, iridium tetrachloride, theammonium complexes such as ammonium chloroiridate, iridium tribromide,iridium trichloride and ammonium chloroiridite. It is preferred to usechloroiridic acid as the source of iridium for the preparation of thesupported catalysts. In general, the iridium component can beimpregnated either prior to, simultaneously with, or after the platinumor palladium metal component is added to the support. However, it hasbeen found that best results are achieved when the iridium component isimpregnated simultaneously with the platinum or palladium component. Apreferred impregnation solution contains chlorophlatinic acid andchloroiridic acid.

The combined weight of the iridium component plus the platinum and/orpalladium component in the catalytic composite may be within the rangeof from about 0.1% to about 4% by weight, preferably from about 0.2% toabout 2.0% by weight and, more preferably, from about 0.3% to about 1.0%by weight, calculated on an elemental basis.

Typical examples of alkyl amine bases which can be utilized in theprocess of this invention are represented by the following structuralformula:

wherein R, R, and R are selected from the group consisting of hydrogen,alkyl containing from 1 to 6 carbon atoms, preferably 1 to 2 carbonatoms and cycloalkyl, provided, however, that not more than two R groupsare hydrogen and R and R can, together with the nitrogen atom, form aheterocyclic ring containing from 5 to 6 aliphatic carbon atoms. Typicalexamples of such amines are methyl amine, ethyl amine, t-butyl amine,diethyl amine, triethyl amine, tri-n-butyl amine, cyclohexyl amine, andpiperidine.

As set forth above, it is preferred that the iridiumalumina containingsupport contain at least one platinum group metal selected from platinumand palladium. The platinum and palladium component may exist within thefinal unreduced catalyst at least in part as a compound such as anoxide, sulfide, halide and the like, or in the elemental state.Generally, the amount of the platinum or palladium component present inthe final catalyst is small compared to the quantities of the othercomponents combined therewith. The platinum or palladium generally hasfrom about 0.5% to about 3.0%, preferably from about 0.10% to about1.0%, by weight of the catalyst calculated on an elemental basis.

Excellent results are obtained when the catalyst contains from 0.2% toabout 0.9% by weight of the platinum group metal.

The platinum or palladium may be incorporated in the catalyst by thecontacting of the alumina support and/or alumina hydrogel and/or aluminasol or gel at any stage in its preparation, either after or before calcination of the alumina hydrogel. The preferred method for adding theplatinum or palladium to the alumina support involves the utilization ofa water-soluble compound of the platinum or palladium to impregnate thealumina support prior to calcination. For example, platinum may be addedto the support by comingling the uncalcined inorganic oxide support withan aqueous solution of chlorophlatinic acid. Other watersolublecompounds of platinum may be employed as impregnation solutions,including, for example, ammonium chloroplatinate and platinum chloride.The utilization of a platinum-chlorine compound, such as chlorophlatinicacid, is preferred since it facilitates the incorporation of both theplatinum component and a minor quantity of a halogen component.

An optional component of the catalyst of the present invention is ahalogen component. Although the precise chemistry of the association ofthe halogen component with the inorganic oxide support is not entirelyknown, it is customary in the art to refer to the halogen component asbeing combined with the support, or with the other ingredients of thecatalyst. This combined halogen may be fluorine, chlorine, bromine, andmixtures thereof. Of these, fluorine and, particularly, chlorine arepreferred for the purposes of the present invention. The halogen may beadded to the support in any suitable manner, either during preparationof the support, or before or after the addition of the catalyticallyactive metallic components. For example, at least a portion of thehalogen may be added at any stage of the preparation of the support, orto the calcined catalyst support, as an aqueous solution of an acid suchas hydrogen fluoride, hydrogen chloride. hydrogen bromide and the likeor as a substantially anhydrous gaseous stream of thesehalogen-containing components. The halogen component, or a portionthereof, may be composited with the support during the impregnation ofthe latter with the platinum group component and/or iridium component,for example, through the utilization of a mixture of chlorophlatinicacid and/or chloroiridic acid and hydrogen chloride. In anothersituation, an alumina hydrosol which can be utilized to form the aluminacomponent may contain halogen and thus contribute at least a portion ofthe halogen component to the final composite. For purposes of thepresent invention, when the catalyst support, for example, aluminaderived from hydrous alumina, e.g., alumina monohydrate, is used in theform of an extrudate, and platinum is added before extrusion, it ispreferred to add the major portion of the halogen component to theotherwise fully composited calcined catalyst by contacting this catalystwith a substantially anhydrous stream of halogen-containing gas. Whenthe catalyst is prepared by impregnating calcined, formed alumina, forexample, spheres produced by the oil drop method, it is preferred toimpregnate the support simultaneously with the platinum group metal,iridium component and halogen. In any event, the halogen will be addedin such a manner as to result in a fully composited catalyst thatcontains from about 0.1% to about 1.5% and preferably from about 0.5% toabout 1.3% by weight of halogen calculated on an elemental basis. Duringprocessing, i.e., the period during which hydrocarbon is beingconverted, the halogen content of the catalyst can be maintained at orrestored to the desired level by the addition of halogen-containingcompounds, such as carbon tetrachloride, ethyl trichloride, t-butylchloride and the like, to the hydrocarbon charge stock before enteringthe reaction zone.

The alumina-containing support utilized in the process of this inventioncomprises a major amount of alumina. Any of the forms of aluminasuitable as supports, for example, in reforming, can be utilized.Furthermore, alumina can be prepared by a variety of methodssatisfactory for the purposes of this invention. The preferred aluminamaterial should be a porous, adsorptive support having a surface area offrom about m /gm. to about 600 m /gm. or more. The alumina comprises amajor proportion, preferably at least about 80%, and more preferably atleast 90%, by weight of the catalyst. The more preferred catalystsupport, or base, is an alumina derived from hydrous alumina,particularly gamma alumina, when formed as pellets and calcined, has anapparent bulk density of from about 0.40 gm./cc. to about 0.85 gm./cc.,pore volume of from about 0.45 ml./gm. to about 0.55 ml./gm., andsurface area of from about 100 m lgm. to about 500 m /gm. As statedabove, the alumina support may contain, in addition, minor proportionsof other well-known refractory inorganic oxides such as silica,zirconia, magnesia and the like. However, the preferred support issubstantially pure alumina derived from hydrous alumina.

The alumina support may be synthetically prepared in any suitable mannerand may be activated prior to use by one or more treatments includingdrying, calcination, steaming and the like. The alumina may be formed inany desired shape such as spheres, pills, cakes, extrudates, powers,granules and the like. For example, the alumina base can be shaped intospheres by means of the well-known oil-drop method. If the spheroidalparticles are calcined, the resulting product is a hard, porous aluminagel. When the catalyst used in the present invention is to be made inthe form of spheres by means of the oil-drop method, it is preferred toadd the additional essential components of the catalyst, i.e., platinumgroup metal, iridium and, optionally, halogen, after calcination of thespheroidal particles.

The catalyst prepared by the process of this invention can be utilizedin a wide variety of processes which include reforming, hydrocracking,isomerization, dehydrogenation, hydrogenation, desulfurization,cyclization, alkylation, polymerization, cracking, hydroisomerizationand the like. In many cases, the processes using these catalysts involvethe simultaneous occurrence of more than one reaction. An example ofthis type of process is reforming, wherein a hydrocarbon feed streamcomprising paraffins and naphthenes is subjected to conditions whichpromote dehydrogenation of naphthenes to aromatics, dehydrocyclizationof paraffins to aromatics, isomerization of paraffins and naphthenes,hydrocracking of naphthenes and paraffins and the like reactions toproduce a high octane or aromatic-rich product stream. Another exampleof process in which more than one reaction occurs simultaneously ishydrocracking, wherein catalysts are used to promote selectivehydrogenation and cracking of high molecular weight unsaturatedmaterials, selective hydrocracking of high molecular weight materialsand other like reactions to produce a generally lower boiling, morevaluable product stream. Yet another example of a process utilizingthese catalysts is an isomerization process, wherein, for example, ahydrocarbon fraction which is relatively rich in straight-chain paraffincomponents is contacted with the catalyst to produce an output streamrich in isoparaffin compounds.

When the catalyst described herein is used in a reforming operation, thereforming system may comprise a reforming zone containing at least onefixed bed of the catalyst previously characterized. This reforming zonemay be one or more separate reactors with suitable heating means therebetween to compensate for the net endothermic nature of the reactionsthat take place in each catalyst bed. The hydrocarbon feed stream thatis charged to the reforming system will comprise hydrocarbon fractionscontaining naphthenes and paraffins that boil within the gasoline range.A-preferred class of charge stocks includes straight run gasolines,natural gasolines, synthetic gasolines and the like. On the other hand,it is frequently advantageous to charge thermally or catalyticallycracked gasolines or higher boiling fractions thereof, called heavynaphthas. Mixtures of straight run and cracked gasolines can also beused to advantage. The gasoline charge stock may be a full boiling rangegasoline having an initial boiling point of from about 50F. to about150F. and an end boiling point within the range of from about 325F. toabout 425F., or may be a selected fraction thereof which generally willbe a higher boiling fraction commonly referred to as a heavy naphtha forexample, a naphtha boiling in the range of about C to about 400F. Insome cases, it is also advantageous to charge pure hydrocarbons ormixtures of hydrocarbons that have been extracted from hydrocarbondistillates for example, a straight-chain paraffin which are to beconverted to aromatics. It is preferred that these charge stocks betreated by conventional pretreatment methods, if necessary, to removesubstantially all sulfurous and nitrogenous contaminants therefrom.

In the reforming embodiment of the present invention, the pressureutilized is selected in the range of from about 50 psig. to about I000psig., with the preferred pressure being from about psig. to about 600psig. Reforming operations may be conducted at the more preferredpressure range of from about 200 psig. to about 400 psig. to achievesubstantially increased catalyst life before regeneration.

For optimum reforming results, the temperature in the reaction zoneshould preferably be within the range of from about 700F. to about 1100F. more preferably in the range of from about 800F. to about 1050F.The initial selection of the temperature within this broad range is madeprimarily as a function of the desired octane of the product reformate,considering the characteristics of the charge stock and of the catalyst.The temperature may then be slowly increased during the run tocompensate for the inevitable deactivation that occurs, to provide aconstant octane product.

In accordance with the reforming process of the present invention,sufficient hydrogen is supplied to provide from about 2.0 to about 20moles of hydrogen per mole of hydrocarbon entering the reaction zone,with excellent results being obtained when from about 5 to about 10moles of hydrogen are supplied per mole of hydrocarbon charge stock.Likewise, the weight hourly 7 space velocity, i.e., WHSV, used inreforming may be in the range from about 0.5 to about 10.0 with a valuein the range from about 2.0 to about 5.0 being preferred. In addition,the catalyst can be sulfided prior to contact with the feed in thereaction zone utilizing conventional presulfiding processes.

The following examples illustrate more clearly the processes of thepresent invention. However, these illustrations are not to beinterpreted as specific limitations on this invention.

EXAMPLE I A commercially available gamma-alumina was utilized to preparethe platinum-iridium supported alumina-containing catalyst. The aluminahad a pore volume of l cc./gm., a density of 0.50 gm./cc., a surfacearea of 240 sq. meters and a diameter of V2 in. The alumina was intheform of an extrudate.

A solution oldeionized water containing 6.87 ml. of chloroplatinic acid(5o.l7 mg. platinum per ml.), 2.93 ml. of chloroiridic acid (37.43 mg.iridium per ml.) and 5 ml. of dilute hydrochloric acid was adjusted to apH of5.5 with ammonium hydroxide. The gamma-alumina (117 gm.) was vacuumimpregnated at a temperature Catalyst ml. of chloroiridic acid (37.43mg. iridium per ml.), 6 ml. of dilute hydrochloric acid, and 2 mg. ofammo nium nitrate was prepared. The gamma-alumina (117 gm.) was vacuumimpregnated at a temperature of 190F. with the iridium-containingaqueous solution. The impregnated alumina was maintained at ambienttemperature for a period of 20 hours. The impregnated alumina was vacuumdried until free flowing and further dried for a period of 5 hours at atemperature of 230F. The platinum-iridium alumina was calcined for aperiod of 3 hours at a temperature of 900F. in a stream of dry air. Theresulting platinum-iridium alumina composite was contacted with hydrogenfora period of 16 hours at 900F. and atmospheric pressure. The catalystcontained 0.33 weight /0 platinum and 0.11 weight /n iridium.

The catalysts of Examples I through IV were performance tested asreforming catalysts by contacting the catalysts with a typicalmid-continent naphtha feedstock at a temperature ol about )50F., WHSV of4, a pressure of 300 psig.. and a mole ratio ol hydrogen to hydrocarbonof from 3: l. The catalyst was subjected to hydrocarbon reforming forabout 150 hours. The results of these tests are as follows:

Example ll Example IV Example l Example lll lnitial Research Octane Number (clear) Aging Rate, Research Octane Number per I00 hours StandardAging Rate* Research Octane Number per 100 hrs.

aging behavior.

of about 190F. with the iridium-containing aqueous solution. Theimpregnated alumina was maintained at ambient temperature for a periodof 20 hours. The iridium-alumina composite was vacuum dried until freeflowing and further dried for a period of 24 hours at a temperature of230F. The platinum-iridium alumina was calcined for a period of 3 hoursat a temperature of 900F. in a stream of dry air. The resultingplatinumiridium alumina composite was contacted with hydrogen for aperiod of 16 hours at 900F. and atmospheric pressure. The catalystcontained 0.33 weight platinum and 0.11 weight iridium.

EXAMPLE II A platinum-iridium alumina composite was manufacturedaccording to the process of Example I, except the pH of the liquidmedium was maintained at 1.0. The resulting platinum-iridium aluminacomposite contained 0.31 weight platinum and 0.10 weight iridium.

EXAMPLE III A platinum-iridium alumina composite was manufacturedaccording to the process of Example 1, except the ammonium hydroxide wasadded until the pH of the liquid medium was 9.0. The resultingplatinum-iridium alumina composite contained 0.34 weight platinum and(HI weight "/n iridium.

EXAMPLE IV A solution of deionized water containing 6.87 ml. ofchloroplatinic acid (56.17 mg. platinum per ml.), 2.93

The above results demonstrate a significant increase in the stability ofthe iridium supported catalyst prepared by the process of the presentinvention. Thus, the standard aging rate l'orthe catalyst prepared bythe process of this invention, e.g., Example I, was only 0.9, ascompared to significantly higher aging rates of platinum-iridiumcatalyst prepared at pHs of l and 9, respectively. In addition, acomparison of the aging rate of Examples I and IV demonstrates theimportance of the base material which is utilized in the process of thisinvention. Thus, the use of ammonium as a source of ammonium ion incomparison to ammonium hydroxide showed a significantly higher agingrate.

While this invention has been described with respect to various specificexamples and embodiments. it is to be understood that the invention isnot limited thereto and that it can be variously practiced within thescope of the following claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

l. A process for the preparation of an iridium-om alumina catalyst whichcomprises impregnating an alumina-containing support with a liquidmedium containing a soluble iridium compound, maintaining the pH of theliquid medium during impregnation of the support in the range of from 3to 6 with a base selected from ammonium hydroxide. a lower alkyl amineand mixtures thereof to yield an iridium alumina composite comprisingfrom about 0.01 to about 1 weight ofiridium and subjecting the resultingiridium compositions to drying, calcination and reduction.

10 platinum compound is present at a concentration to yield aluminacomposite comprising from about 0.05 to about 1.0 weight platinum.

8. A process of claim 4 wherein the platinum is present at aconcentration of from about 0.10 to about 1 weight and the iridium ispresent at a concentration of about 0.025 to about 0.50 weight 72.

9. A process of claim 3 wherein the pH of the liquid medium is fromabout 3.5 to 5.

10. A process of claim 8 wherein the pH of the liquid

1. A PROCESS FOR THE PREPARATION OF AN IRIDIUM-ON-ALUMINA CATALYST WHICHCOMPRISES IMPREGNATING AN ALUMINA-CONTAINING SUPPORT WITH A LIQUIDMEDIUM CONTAINING A SOLUBLE IRIDIUM COMPOUND, MAINTAINING THE PH OF THELIQUID MEDIUM DURING IMPREGNATION OF THE SUPPORT IN THE RANGE OF FROM 3TO 6 WITH A BASE SELECTED FROM AMMONIUM HYDROXIDE, A LOWER ALKYL AMINEAND MIXTURES THEREOF TO YIELD AN IRIDIUM ALUMINA COMPOSITE COMPRISINGFROM ABOUT 0.01 TO ABOUT 1 WEIGHT % OF IRIDIUM AND SUBJECTING THERESULTING IRIDIUM COMPOSITIONS TO DRYING, CALCINATION AND REDUCTION. 2.A process of claim 1 wherein the liquid medium contains a decomposableplatinum compound and the alumina-containing support is an aluminaderived from hydrous alumina.
 3. A process of claim 1 wherein the baseis ammonium hydroxide.
 4. A process of claim 2 wherein the base isammonium hydroxide.
 5. A process of claim 2 wherein the iridium compoundis chloroiridic acid.
 6. A process of claim 4 wherein the iridiumcompound is chloroiridic acid.
 7. A process of claim 2 wherein thedecomposable platinum compound is present at a concentration to yieldalumina composite comprising from about 0.05 to about 1.0 weight %platinum.
 8. A process of claim 4 wherein the platinum is present at aconcentration of from about 0.10 to about 1 weight % and the iridium ispresent at a concentration of about 0.025 to about 0.50 weight %.
 9. Aprocess of claim 3 wherein the pH of the liquid medium is from about 3.5to
 5. 10. A process of claim 8 wherein the pH of the liquid medium isfrom about 3.5 to 5.